This invention relates to a process for pre-heating a vitrifiable batch prior to its introduction into a glass-melting furnace, in which pre-heating use is made of hot gases from the melting furnace. The invention also relates to apparatus for carrying out this process.
It is known to pre-heat a vitrifiable batch during its feed to a glass-melting furnace by feeding the batch in counter-current to hot gases escaping from the furnace. The gases thereby pre-heat the batch by heat-exchange.
The known processes can be roughly divided into three categories. In known processes of a first category the batch is allowed to fall freely in a shaft which also serves as a chimney for gases escaping from the furnace. The batch is thus heated by heat-exchange as it falls towards the furnace entrance. Such processes are not considered to be satisfactory for a number of reasons. One reason is the low heat recovery due to the brevity of the heat-exchange contact between the batch and the hot gases. A second objection to such processes is the fact that the batch entering the furnace tends to be of uncontrolled variable composition due to the segregation of batch constituents of different sizes and specific gravities during their fall. This lack of uniformity in the feed to the furnace tends to be greater when there is a longer fall path of the batch in heat exchange relationship to the hot gases.
The second category of known processes comprises those in which the batch forms a column in a feed shaft through which the hot gases can pass via the labyrinth of spaces between the individual batch pieces or particles. This type of feed system cannot be used for batches in powder form because the column of powder is substantially impervious to the furnace gases. This feed system is only applicable for feeding batches in the form of granules or pieces large enough to ensure that the batch column is pervious to the furnace gases. Experience shows that even when the batch forms a gas-pervious column, the feed to the furnace is not satisfactory. The feed of batch to the furnace is irregular and substantial variation in the temperature to which different parts of the batch are pre-heated cannot be avoided. These irregularities render it very difficult to sustain given quality standards in the production and conditioning of molten glass in the furnace.
In the known processes of the third category, the batch is delivered into a feed shaft wherein there are baffle walls past which the furnace gases can flow and which are constructed and arranged to catch at least some of the delivered batch. The baffle walls are constructed and arranged so that any batch material falling thereon is retained until, due to melting of batch constituents, it acquires a flowable condition and can flow along the baffle walls and pour over the edges thereof, into the furnace. With this system it should be possible to avoid undesirable variations in course of time in the pre-heated condition of the batch entering the furnace. However, in practice the feed rate is very irregular. It has proved to be impossible to prevent interruptions in the flow of batch off the baffle walls due to batch material thereon becoming sintered or retained at positions where it is not adequately exposed to the hot gases with the result that such material forms a barrier obstructing the continuous flow of material along the baffle walls.